Melanex is an anesthetic carried in 2260s Starfleet medkits.
In 2269, Doctor McCoy used melanex to make Spock look ill, fooling the computer which controlled the Shore Leave Planet. McCoy described the effects of melanex as, "brief unconsciousness and temporary skin discoloration", adding that "it looks worse than it is."
Once the doctor had administered the Vulcan with 10 ccs of melanex by hypospray, McCoy told Spock that it took ten minutes to take effect, before rendering the Vulcan unconscious for five minutes (TAS: "Once Upon a Planet").